1. Field of the Invention
The present invention relates to a secondary suspension for railway vehicle three-axle trucks and more particularly to an improved secondary suspension utilizing laminated pads.
2. Description of the Prior Art
The secondary suspension of a railway truck is a spring system that divides the truck mass from the chassis mass. It is possible to utilize secondary springs that will also accommodate the necessary truck rotation relative to the chassis and lateral motion of the truck relative to the chassis. Such a system for a three axle truck is described in U.S. Pat. No. 3,451,355, to Dobson et al, which uses four laminated rubber-and-metal elements, such as made by Metalastik of Leicester, England, and others. Rubber-and-metal laminated elements are used in a similar fashion in applicants' Canadian Patent 929,798 entitled "Two-Axle Truck Assembly."
In the Dobson et al patent it is noted that the two inboard pads take approximately one-sixth of the chassis load, whilst the two outboard pads one-third of the chassis load. Therefore, if the two inboard pads were added together to take one-third of the load, then obviously one single pad identical to the outboard pads would suffice. Therefore, by positioning a single inboard pad centrally and aligned between the two outboard pads, the two inboard pads are eliminated. A three-pad arrangement versus the four-pad arrangement gives substantially the same stability of truck relative to chassis for lateral leaning and in the fore and aft stability of the truck.
The three-pad arrangement of the present invention is designed to give better flange forces, i.e. lower flange forces, because of the torsional restoring rate of the rubber suspension in the following manner: The rubber pads, when loaded in the perpendicular direction, lose shear stiffness as the load is increased; therefore, two pads operating at, say, 20% of their ultimate capacity will have a given shear stiffness which the truck must work against to rotate when negotiating a curve. However, the same pad when loaded to 40% of its ultimate capacity will have a substantially lower shear stiffness than at 20%. Further, one pad loaded to 40%, already lower than one pad loaded to 20%, is obviously 50% fewer pads. In other words, a twofold reduction is achieved, (1) a decrease in the shear stiffness by increasing load on a pad and (2) decreasing the number of pads to be sheared.
It is known from theoretical analysis and from the system described in the above-mentioned patent, that the shear rates of the rubber-and-metal pads which act to align the truck with the chassis contribute adversely to flange loading as the locomotive negotiates curves; and further that the same torsional restoring rate (summation of all shear rates times radius) contributes to dynamic instability on straight track.
It is also known from theoretical analysis that a very low torsional rate is necessary when the locomotive trucks are equipped with an inter-bogie control device, a device or arrangement well known in the art for controlling truck alignment and flange forces when negotiating curves.